<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>计算属性和侦听器</title>
    <script type="text/javascript" src="../js/vue.js"></script>
    <script src="https://cdn.jsdelivr.net/npm/axios@0.12.0/dist/axios.min.js"></script>
    <script src="https://cdn.jsdelivr.net/npm/lodash@4.13.1/lodash.min.js"></script>
</head>
<body>
    <!-- 计算属性缓存 vs 方法
        计算属性是基于它们的响应式依赖进行缓存的：只在相关响应式依赖发生改变时它们才会重新求值。
        这就意味着只要 message 还没有发生改变，多次访问 reversedMessage 计算属性会立即返回之前的计算结果，而不必再次执行函数。
    -->
    <div id="example">
        <!-- 计算属性 -->
        <p>Original message: "{{ message }}"</p>
        <p>Computed reversed message: "{{ reversedMessage }}"</p>
        <p>Computed now: "{{ now }}"</p>
        <!-- 方法 -->
        <p>Computed reversed message: "{{ reversedMessage1() }}"</p>
    </div>
    

    <!-- 计算属性 vs 侦听属性 -->
    <div id="demo">{{ fullName }}</div>
    <div id="demo1">{{ fullName }}</div>

    <!-- 侦听器 -->
    <div id="watch-example">
        <p>
          Ask a yes/no question:
          <input v-model="question">
        </p>
        <p>{{ answer }}</p>
    </div>
</body>
    <script>
        var vm = new Vue({
            el: '#example',
            data: {
                message: 'Hello'
            },
            computed: {
                // 计算属性的 getter
                reversedMessage: function () {
                    // `this` 指向 vm 实例
                    return this.message.split('').reverse().join('')
                },
                now: function () {
                    return Date.now()
                }
            },
            // 在组件中
            methods: {
                reversedMessage1: function () {
                    return this.message.split('').reverse().join('')
                }
            }
        })

        var vm = new Vue({
            el: '#demo',
            data: {
                firstName: 'Foo',
                lastName: 'Bar',
                fullName: 'Foo Bar'
            },
            watch: {
                firstName: function (val) {
                    this.fullName = val + ' ' + this.lastName
                },
                lastName: function (val) {
                    this.fullName = this.firstName + ' ' + val
                }
            }
        })

        var vm = new Vue({
            el: '#demo1',
            data: {
                firstName: 'Foo',
                lastName: 'Bar'
            },
            computed: {
                // 默认为getter方法，获取时被调用
                fullName: function () {
                    return this.firstName + ' ' + this.lastName
                },
                fullName: {
                    // getter
                    get: function () {
                        return this.firstName + ' ' + this.lastName
                    },
                    // setter 运行 vm.fullName = 'John Doe' 时，setter 会被调用
                    set: function (newValue) {
                        var names = newValue.split(' ')
                        this.firstName = names[0]
                        this.lastName = names[names.length - 1]
                    }
                }
            }
        })

        var watchExampleVM = new Vue({
            el: '#watch-example',
            data: {
                question: '',
                answer: 'I cannot give you an answer until you ask a question!'
            },
            watch: {
                // 如果 `question` 发生改变，这个函数就会运行
                question: function (newQuestion, oldQuestion) {
                    this.answer = 'Waiting for you to stop typing...'
                    this.debouncedGetAnswer()
                }
            },
            created: function () {
                // `_.debounce` 是一个通过 Lodash 限制操作频率的函数。
                // 在这个例子中，我们希望限制访问 yesno.wtf/api 的频率
                // AJAX 请求直到用户输入完毕才会发出。想要了解更多关于
                // `_.debounce` 函数 (及其近亲 `_.throttle`) 的知识，
                // 请参考：https://lodash.com/docs#debounce
                this.debouncedGetAnswer = _.debounce(this.getAnswer, 500)
            },
            methods: {
                getAnswer: function () {
                    if (this.question.indexOf('?') === -1) {
                        this.answer = 'Questions usually contain a question mark. ;-)'
                        return
                    }
                this.answer = 'Thinking...'
                var vm = this
                axios.get('https://yesno.wtf/api')
                    .then(function (response) {
                        vm.answer = _.capitalize(response.data.answer)
                    })
                    .catch(function (error) {
                        vm.answer = 'Error! Could not reach the API. ' + error
                    })
                }
            }
        })
    </script>
</html>